1. Field of the Invention
The invention relates to the recovery of the more readily adsorbable component of a gas mixture, particularly the recovery of nitrogen from air. More particularly, it relates to the recovery of nitrogen from air using an improved pressure swing adsorption process and apparatus.
2. Description of the Prior Art
In numerous chemical processing, refinery, metal production and other industrial applications, high purity nitrogen is desired for purging, blanketing, the providing of metal treating atmospheres, and the like. Enriched oxygen gas is also frequently required for various purposes in the same facility. Nitrogen and oxygen can, of course, be obtained by various known techniques for air separation. Pressure swing adsorption (PSA) processing is particularly suited for such air separation in a variety of applications, particularly in relatively small sized operations for which the use of a cryogenic air separation plant may not be economically feasible. Typical applications of this type require purities in the range of 95.0-99.9% nitrogen at flow rates of up to 100,000 cubic feet per hour.
In the PSA process, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component are commonly passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. The bed is thereafter depressured to a lower desorption pressure for desorption of said more readily adsorbable component and its removal from the bed prior to the introduction of additional quantities of the feed gas mixture to the bed as cyclic adsorption-desorption operations are continued in said bed. As those skilled in the art will readily appreciate, the PSA process is commonly employed in multi-bed systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds in the system.
As applied for air separation purposes, PSA systems achieve the desired separation of oxygen and nitrogen because of the greater selectivity of the adsorbent employed for either oxygen or nitrogen. The adsorptive capacity of the particular adsorbent material employed increases at higher pressure levels and decreases at lower pressures. In PSA processes and systems for the production of high purity nitrogen product, the adsorbent employed may be one having a greater selectivity for either the desired nitrogen product or for oxygen. In systems in which the adsorbent employed is an oxygen selective material, such as carbon molecular sieves, the product nitrogen is produced as the less readily adsorbable component during the passage of feed air to bed of adsorbent at a higher adsorption pressure. In systems in which the adsorbent employed is a nitrogen selective material, such as zeolite molecular sieves, the product nitrogen is produced as the more readily adsorbable component upon the depressurization of the adsorbent bed to its lower desorption pressure.
There have been numerous attempts to enhance the PSA process and system, particularly to lower capital costs, increase reliability and minimize operating costs, as by achieving relatively low power consumption per unit of product being produced. One desirable goal in the achieving of such overall objectives is to enable the production of relatively high purity coproduct in addition to the desired high purity product. As applied to air separation and other gas separation operations, the Batta patent, U.S. Pat. No. 3,636,679, discloses a PSA cycle as applied to two or more beds wherein each bed is partially repressurized from a lower desorption pressure by simultaneous feed gas-product gas introduction from opposite ends of the bed followed by further repressurization to higher adsorption pressure by feed gas alone, after which the bed is cocurrently depressurized with release of less readily adsorbable component from the discharge end thereof, a portion thereof being recovered as product gas and the remainder being used for pressure equalization and providing purge gas to another bed or beds in the system. The bed is then countercurrently depressurized with release of gas from the feed end of the bed and purged prior to commencing partial repressurization using additional feed gas as cyclic operations are carried out on a continuous basis. The approach of this patent has been successfully employed in air separation operations intended to recover product oxygen as the less readily adsorbable component of air. The Batta process is not applicable, however, to the recovery of the more readily adsorbable component of air, e.g., nitrogen, as the desired high purity product gas.
Various other processes exist, however, in which it is desired to recover the more readily adsorbable component as product gas. Such processes commonly employ a vacuum cycle in which the more readily adsorbable component of the gas mixture is desorbed from the bed at a subatmospheric desorption pressure. Thus, the Tamura patent, U.S. Pat. No. 3,797,201, discloses an air separation process that includes the introduction of air at atmospheric adsorption pressure into an adsorbent bed capable of selectively adsorbing the more readily adsorbable nitrogen component thereof, followed by vacuum desorption to recover said nitrogen as desired product gas. To increase the purity of the product nitrogen, Tamura teaches the carrying out of the initial adsorption step with release of oxygen-rich gas from the discharge end thereof until breakthrough of the nitrogen adsorption front at said discharge end of the bed and the incorporation of a cocurrent purge at said higher adsorption pressure, using nitrogen for purge, prior to countercurrent vacuum desorption and repressurization. The application of this process tends to be limited by the unavailability of coproduct oxygen at a useable pressure and in an energy efficient manner, although high purity nitrogen product can be obtained thereby. A similar processing cycle is described in the Sircar et al. patents, U.S. Pat. Nos. 4,013,429 and 4,264,340, said cycle employing two adsorption trains, each consisting of a pretreatment bed and a main separation bed, together with variable volume surge tanks to accommodate discontinuous flow rates between processing steps. The high degree of vacuum required during desorption and the overall complexity of the process, however, serve to add significantly to the equipment and power costs associated with this process.
Vacuum desorption is likewise employed in the process of the Miwa et al. patent U.S. Pat. No. 4,070,164, which includes pretreatment for cleaning and drying air and a processing cycle that includes (1) pressurization of a bed to about 4 atm by air feed, (2) cocurrent purge at said elevated pressure with nitrogen to remove an oxygen rich stream from the discharge end of the bed, (3) countercurrent depressurization of nitrogen rich gas from the feed end of the bed, and (4) vacuum desorption to about 0.3 atm with release of additional nitrogen-rich gas from said feed end of the bed. By the combining of gas released during the two countercurrent depressurization steps, a constant flow of high purity nitrogen is recovered from the system, although the recovery level for the desired nitrogen is quite low using this approach. Both capital and operating costs are relatively high for this processing cycle carried out in four bed systems. Capital costs are high because of the use of four adsorbent vessels, with associated valve, compression equipment and other requirements. Operating costs tend to be high because of the relatively low efficiency of the process. The same four processing steps were also disclosed in the Armond patent, U.S. Pat. No. 4,129,424, which also provides for the cocurrent purge step to be carried out at a pressure substantially equal to the partial pressure of the nitrogen in the feed gas, thereby significantly reducing the amount of purge gas required to saturate the bed as compared with similar processes in which purging is carried out at a higher pressure. This, in turn, leads to the inclusion of a cocurrent venting step after air feed introduction to reduce the pressure of the bed to that of the purge gas.
The art of recovering high purity nitrogen from air by use of PSA processing was advanced by the improved process disclosed in the Werner et al. patent, U.S. Pat. No. 4,599,094. This process includes the steps of (1) pressurization, (2) copurge, and (3) countercurrent depressurization to a lower subatmospheric desorption pressure, with the recovery and purity of nitrogen as the more readily adsorbable component of air being enhanced. A portion of the coproduct effluent gas released from the bed upon copurge with nitrogen at elevated pressure is recovered as oxygen, i.e., less readily adsorbable component, coproduct gas, while an additional portion of said oxygen is introduced to the discharge end of a bed being repressurized by the introduction of feed air to the feed end of said bed. A third portion of such oxygen is introduced to the feed end of a bed after the bed has been at least partially repressurized. A portion of the nitrogen released from the feed end of the bed upon countercurrent depressurization is employed as said nitrogen copurge gas.
The Werner et al. process enables high purity nitrogen to be recovered at a high recovery level, with enriched oxygen coproduct being recovered at a relatively high recovery level. Nevertheless, the complexity of the process and the high compression ratios and through put requirements of the system results in relatively high capital and operating costs for this processing approach.
Despite such efforts in the art, those skilled in the art will appreciate that a need remains for the development of improved PSA processing for the production of nitrogen as the more readily adsorbable component of air, wherein the desired nitrogen product can be recovered at high purity levels at relatively low capital and operating costs. Such improvement would enhance the ability of the highly desirable pressure swing adsorption technology to satisfy the need for high purity, low cost nitrogen for a variety of practical, commercially desirable industrial applications.
It is an object of the invention, therefore, to provide an improved PSA process and system for the production of high purity nitrogen.
It is another object of the invention to provide a simplified PSA process and system for the production of high purity nitrogen from air.
It is another object of the invention to provide a PSA process and system capable of minimizing the capital costs, power consumption, and overall costs of recovering high purity nitrogen product from air.
It is a further object of the invention to provide an improved PSA process and system for the recovery of the more readily adsorbable component of a feed gas mixture as a desired high purity product gas.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.